A known developing cartridge includes a window member. The developing cartridge is provided at an outer surface of a casing with a detection window constituted by a window member. The detection window is for detecting a residual amount of developing agent from outside the developing cartridge. For example, the developing cartridge includes a first frame and a second frame. The first frame is a container-like frame with an upper side thereof open and configured to accommodate or contain toner. The second frame is used as a cover or lid for the first frame. A side wall of the first frame has an opening. The window member engages the opening. The developing cartridge includes a rotatable agitator in the casing. An amount of developing agent floated or flying in the developing cartridge when the agitator rotates, may be detected via the window member.